A ferritic stainless steel designated as Type 430F has been used in magnetic devices such as cores, end plugs, and housings for solenoid valves. A commercially available composition of Type 430F alloy contains, in weight percent 0.065% max. C, 0.80% max. Mn, 0.30-0.70% Si, 0.03% max. P, 0.25-0.40% S, 17.25-18.25% Cr, 0.60% max. Ni, 0.50% max. Mo, and the balance is essentially Fe. Type 430F alloy provides a good combination of magnetic properties, machinability, and corrosion resistance. Although Type 430F alloy provides good corrosion resistance in such mild environments as air having relatively high humidity, fresh water, foodstuffs, nitric acid, and dairy products, the alloy's ability to resist corrosion in chloride-containing environments leaves much to be desired.
Type 430FR alloy is a ferritic stainless steel that is similar in composition to Type 430F alloy except for higher silicon, i.e., 1.00-1.50% Si. Type 430FR alloy provides higher electrical resistivity and higher annealed hardness than Type 430F alloy. However, Type 430FR provides corrosion resistance that is about the same as Type 430F alloy.
A need has arisen for a soft magnetic, easily machinable alloy that provides better corrosion resistance in chloride-containing environments than either Type 430F alloy or Type 430FR alloy. Although it is known that molybdenum benefits the corrosion resistance of some stainless steels, e.g., the so-called 18Cr-2Mo steel alloy, in chloride-containing environments, it has been found that the addition of molybdenum alone to a ferritic stainless steel such as Type 430F or 430FR, does not consistently provide the desired level of corrosion resistance in such an environment. Accordingly, it would be desirable to have a soft magnetic, free-machining, ferritic alloy that also provides consistently good resistance to corrosion in a chloride-containing environment.